Lumbar Spine Anatomy

Typical Lumbar Vertebrae

• The body of the typical lumbar vertebra is massive, with a transverse diameter greater than the anterior diameter and height.
• The size and shape reflect the need to support great compressive loads caused by body weight, ground reaction forces, and muscle contraction.
• Laminae and pedicles are short and thick, forming the posterior and lateral walls of the nearly triangular vertebral canal.
• Pedicles are short and thick, project posterolaterally.
• Laminae are short and broad.
• Superior articular (zygapophyseal) facets are moderately concave, facing medial to posterior-medial.
• Inferior articular (zygapophyseal) facets are vertical and convex, facing slightly anteriorly and laterally.
• Mamillary processes, small bumps, are located on the posterior edge of each superior articular facet and serve as attachment for the multifidus and medial intertransverse muscles.

Transverse Process

• The transverse process is long and slender and extends horizontally.
• Accessory processes, small and irregular bony prominences, are located on the posterior surface of the transverse process near its attachment to the pedicle.
• Accessory processes serve as attachment sites for the multifidus and medial intertransverse muscles.

Spinous Process

• The spinous process is broad and thick and extends horizontally.

Vertebral Foramen

• The vertebral foramen is triangular and larger than the thoracic vertebral foramen but smaller than the cervical vertebral foramen.

5th Lumbar Vertebra

• The fifth lumbar vertebra is a transitional vertebra and differs from the rest of the lumbar vertebrae.
• The body of the vertebra is wedge-shaped, with the anterior portion being of greater height than the posterior portion.
• The L5/S1 lumbosacral disk is also wedge-shaped.
• The inferior articular facets of L5 articulate with the superior articular facets of the sacrum.
• Apophyseal joints are typically oriented much closer to the frontal plane than the other lumbar articulations.
• The L5–S1 apophyseal joints provide an important source of anterior-posterior stability to the lumbosacral junction.

Lumbosacral Joint

• The lumbosacral joint is formed by two separate articulations: the anterior intervertebral joint and the apophyseal joints.
• The lumbosacral joint is the most unstable segment of the lumbar spine.

Forces Acting on the Lumbosacral Joint

• The weight (P) can be resolved into two forces: a force (N) acting perpendicular to the upper surface of S1 and a force (G) acting parallel to the upper surface of S1 and pulling L5 anteriorly.
• These forces are transmitted through a single point in the pars interarticularis.

Lumbosacral Angle

• The lumbosacral angle is the angle formed by the first sacral segment and the horizontal.
• The size of the angle varies with the position of the pelvis and affects the superimposed lumbar curvature.
• An increase in this angle will result in an increase in lordosis of the lumbar curve and will increase the amount of shearing stress at the lumbosacral joint.

Ligaments

• Lateral lumbosacral ligament: arises from the lower margin of the transverse process of L5 vertebra and passes obliquely inferiorly to attach on the ala of the sacrum.
• The ligament is partially continuous with the iliolumbar ligament at its origin point and blends with the anterior sacroiliac ligament at its insertion on the sacrum.

Ligament Stability of the Lumbar Spine

• The thoracolumbar fascia consists of three layers: the posterior, middle, and anterior.
• The posterior layer is large, thick, and fibrous and arises from the spinous processes and supraspinous ligaments of the thoracic, lumbar, and sacral spines.